Ice Sheet Response Research Articles

Surface exposure ages imply multiple low-amplitude Pleistocene variations in East Antarctic Ice Sheet, Ricker Hills, Victoria Land

AbstractOne of the major issues in (palaeo-) climatology is the response of Antarctic ice sheets to global climate changes. Antarctic ice volume has varied in the past but the extent and timing of these fluctuations are not well known. In this study, we address the question of amplitude and timing of past Antarctic ice level changes by surface exposure dating using in situ produced cosmogenic nuclides (10Be and 21Ne). The study area lies in the Ricker Hills, a nunatak at the boundary of the East Antarctic Ice Sheet in southern Victoria Land. By determining exposure ages of erratic boulders from glacial drifts we directly date East Antarctic Ice Sheet variations. Erosion-corrected neon and beryllium exposure ages indicate that a major ice advance reaching elevations of about 500 m above present ice levels occurred between 1.125 and 1.375 million years before present. Subsequent ice fluctuations were of lesser extent but timing is difficult as all erratic boulders from related deposits show complex exposure histories. Sample-specific erosion rates were on the order of 20–45 cm Ma-1 for a quartzite and 10–65 cm Ma-1 for a sandstone boulder and imply that the modern cold, arid climate has persisted since at least the early Pleistocene.

Effect of isostasy on dynamical ice sheet modeling: A case study for Eurasia

Isostatic adjustment is an important process when modeling dynamical ice sheet evolution. A number of different types of Earth model are used to model this process, ranging from models with local or elastic lithospheres with a single relaxation time to account for temporal behavior to self‐gravitating spherically symmetric models of the crust‐mantle system. We performed sensitivity tests to assess the dynamical response of ice sheets to isostatic adjustment. The resulting ice sheet geometries depend on the response times of the system, including the growth rate of ice sheets versus the relaxation time of the Earth. Differences in lithospheric strength cause large differences in the modeled ice sheets, with the largest differences in isostatic adjustment in the vicinity of the ice margins. In addition, we compared predictions based on an often used flexural model with an elastic plate and a single relaxation time to a spherically symmetric self‐gravitating viscoelastic approach. No flexural model was able to reproduce the viscoelastic response completely, due to the fact that the models have an intrinsically different spatial and temporal response. When applied to a full transient glacial cycle in Eurasia, even the best fitting flexural model underestimated the ice volume at the LGM by 30 percent. Our results indicate that the commonly used value of 3000 years for the relaxation time in flexural models is too short for Eurasia by at least 3000 years.

The role of subglacial water in ice-sheet mass balance

Subglacial water can significantly affect the velocity of ice streams and outlet glaciers of ice sheets. Depending on the geometry and capacity of the subglacial hydrologic system, increased surface melting in Greenland over the coming decades may influence the ice sheet's mass balance. Furthermore, subglacial lakes in Antarctica can modulate ice velocities and act as nucleation points for new fast-flowing ice streams. In the coming decades, significant changes in the polar regions will increase the contribution of ice sheets to global sea-level rise. Under the ice streams and outlet glaciers that deliver ice to the oceans, water and deformable wet sediments lubricate the base, facilitating fast ice flow. The influence of subglacial water on fast ice flow depends on the geometry and capacity of the subglacial hydrologic system: water moving rapidly through a well-connected system of conduits or channels will have little impact on ice-sheet velocities, but water injected into a spatially dispersed subglacial system may reduce the effective pressure at the base of the ice sheet, and thereby trigger increased ice-sheet velocities. In Greenland, the form of the subglacial hydrologic system encountered by increasing surface melt water will determine the influence of changing atmospheric conditions on ice-sheet mass balance. In Antarctica, subglacial lakes have the capacity to both modulate velocities in ice streams and outlet glaciers and provide nucleation points for new fast ice-flow tributaries. Climate models of ice-sheet responses to global change remain incomplete without a parameterization of subglacial hydrodynamics and ice dynamics.

River ice cover flexure by an incoming wave

Abstract A river's ice cover can be broken up by unsteady flows that may result from a sudden increase in discharge resulting from a dam break event, an ice jam release event or a flash-flood. As the resulting wave (surge or ‘jave’) travels downstream, if it is not strong enough to break the cover, its amplitude and celerity will be dampened whereas if it is, they may be amplified. It is therefore important to know what rise in levels is required to break an intact ice sheet. In this paper, 1D and 2D numerical models are used to investigate the wave–ice cover interaction in order to formulate simple river ice sheet breakup criteria. The 1D model (based on the theory of beams on an elastic foundation) uses an iterative, in-house, finite difference method to converge on a solution that correctly models the limiting effect that the ice sheet's freeboard has on the imposed load. The analyses show the importance of non-linear ice sheet response to increasing wave amplitudes and/or water surface slopes as a function of ice sheet thickness. The paper also presents an analysis of water shear effects on axial and bending stresses. Lastly, a finite element method (FEM) is used to resolve the 2-D problem. For ice sheets frozen into or simply attached to the river banks, computed stresses are shown to be significantly different from the 1-D solution.

A Closer Look at the IPCC Report

A Closer Look at the IPCC Report

Atmospheric transport and concentration of diatoms in surficial and glacial sediments of the Allan Hills, Transantarctic Mountains

A global climate up to 3 °C warmer than present around 3 Ma is widely accepted, but the response of the Antarctic ice sheets at that time is still debated. The presence of rare Pliocene marine diatoms in the Sirius Group tillites of the Transantarctic Mountains has led some to infer that the warming caused a collapse of the ice sheet, exposing the Antarctic interior to inland seas. Here we document an alternative source for the Sirius Group diatoms through atmospheric transport onto the surface of the East Antarctic Ice Sheet to be carried downslope by katabatic winds flowing over the Allan Hills, one of many nunataks that border the East Antarctic Ice Sheet. Evidence comes from diatom remains recovered in windtraps, surficial sediments and Sirius Group deposits themselves. The abundance and preservation of the diatoms being actively transported and trapped in the Allan Hills far exceeds those observed in the Sirius Group. Melting of wind-blown snow during summer provides a mechanism to transport and concentrate these diatoms into “diatom-rich” layers. This mechanism, along with the extreme sparseness and sporadic occurrence of diatoms previously documented in the Sirius Group, suggests a more likely scenario in which diatoms from the atmosphere contaminated tillites that had been deposited beneath wet-based glaciers in much older and warmer times. In this scenario, Pliocene marine diatoms may have been injected into the troposphere by meteorite impact, or picked up from exposed land around the Ross Embayment following the disappearance of the Ross Ice Shelf in Pliocene times. We conclude that the Pliocene age assigned to the Sirius Group on the basis of rare diatoms should be regarded as unproven, and the inferred collapse of the East Antarctic Ice Sheet at that time unlikely.

Rapid sea level rise and ice sheet response to 8,200‐year climate event

The largest abrupt climatic reversal of the Holocene interglacial, the cooling event 8.6–8.2 thousand years ago (ka), was probably caused by catastrophic release of glacial Lake Agassiz‐Ojibway, which slowed Atlantic meridional overturning circulation (AMOC) and cooled global climate. Geophysical surveys and sediment cores from Chesapeake Bay reveal the pattern of sea level rise during this event. Sea level rose ∼14 m between 9.5 to 7.5 ka, a pattern consistent with coral records and the ICE‐5G glacio‐isostatic adjustment model. There were two distinct periods at ∼8.9–8.8 and ∼8.2–7.6 ka when Chesapeake marshes were drown as sea level rose rapidly at least ∼12 mm yr−1. The latter event occurred after the 8.6–8.2 ka cooling event, coincided with extreme warming and vigorous AMOC centered on 7.9 ka, and may have been due to Antarctic Ice Sheet decay.

Major ice sheet response in eastern New England to a cold North Atlantic region, ca. 16–15 cal ka BP

A large ice sheet still covered almost all of Maine and eastern New England until ca. 15 cal ka BP, reaching south of 45 °S, despite rising summer insolation intensity and major ice recession elsewhere outside the North Atlantic region. Furthermore, the well-studied moraine belt along eastern coastal Maine, including the prominent Pineo Ridge delta/moraine complex and Pond Ridge moraine, indicates repeated readvances and stillstands between ca. 16 and 15 cal ka BP. This moraine belt reflects a considerable ice sheet response over eastern North America during this time period, coeval with the latter half of the European Oldest Dryas period. Moraine deposition was concurrent with reduction or elimination of North Atlantic meridional overturning, starting with the earlier onset of peak IRD and Heinrich Event 1 (HE-1). The existing 14C chronology suggests that the coastal moraine belt and the persistence of the ice sheet until ∼ 15 cal ka BP was a response to the severe cooling of the North Atlantic region after ∼ 17 cal ka BP.

Climatic Conditions for modelling the Northern Hemisphere ice sheets throughout the ice age cycle

Abstract. The ice sheet-climate interaction as well as the climatic response to orbital parameters and atmospheric CO2 concentration are examined in order to drive an ice sheet model throughout an ice age cycle. Feedback processes between ice sheet and atmosphere are analyzed by numerical experiments using a high resolution General Circulation Model (GCM) under different conditions at the Last Glacial Maximum. Among the proposed processes, the ice albedo feedback, the elevation-mass balance feedback and the desertification effect over the ice sheet were found to be the dominant processes for the ice-sheet mass balance. For the elevation-mass balance feedback, the temperature lapse rate over the ice sheet is proposed to be weaker than assumed in previous studies. Within the plausible range of parameters related to these processes, the ice sheet response to the orbital parameters and atmospheric CO2 concentration for the last glacial/interglacial cycle was simulated in terms of both ice volume and geographical distribution, using a three-dimensional ice-sheet model. Careful treatment of climate-ice sheet feedback is essential for a reliable simulation of the ice sheet changes during ice age cycles.

Rapid response of modern day ice sheets to external forcing

The great ice sheets covering Antarctica and Greenland were, traditionally, believed to take thousands of years to respond to external forcing. Recent observations suggest, however, that major changes in the dynamics of parts of the ice sheets are taking place over timescales of years. These changes were not predicted by numerical models, and the underlying cause(s) remains uncertain. It has been suggested that regional oceanic and/or atmospheric warming are responsible but separating the influence and importance of these two forcings has not been possible. In most cases, the role of atmospheric versus oceanic control remains uncertain. Here, we review the observations of rapid change and discuss the possible mechanisms, in the light of advances in numerical modelling and our understanding of the processes that may be responsible.

‘Calving laws’, ‘sliding laws’ and the stability of tidewater glaciers

AbstractA new calving criterion is introduced, which predicts calving where the depth of surface crevasses equals ice height above sea level. Crevasse depth is calculated from strain rates, and terminus position and calving rate are therefore functions of ice velocity, strain rate, ice thickness and water depth. We couple the calving criterion with three ‘sliding laws’, in which velocity is controlled by (1) basal drag, (2) lateral drag and (3) a combination of the two. In model 1, velocities and strain rates are dependent on effective pressure, and hence ice thickness relative to water depth. Imposed thinning can lead to acceleration and terminus retreat, and ice shelves cannot form. In model 2, ice velocity is independent of changes in ice thickness unless accompanied by changes in surface gradient. Velocities are strongly dependent on channel width, and calving margins tend to stabilize at flow-unit widenings. Model 3 exhibits the combined characteristics of the other two models, and suggests that calving glaciers are sensitive to imposed thickness changes if basal drag provides most resistance to flow, but stable if most resistance is from lateral drag. Ice shelves can form if reduction of basal drag occurs over a sufficiently long spatial scale. In combination, the new calving criterion and the basal–lateral drag sliding function (model 3) can be used to simulate much of the observed spectrum of behaviour of calving glaciers, and present new opportunities to model ice-sheet response to climate change.

Orbital changes and climate

At the 41,000-period of orbital tilt, summer insolation forces a lagged response in northern ice sheets. This delayed ice signal is rapidly transferred to nearby northern oceans and landmasses by atmospheric dynamics. These ice-driven responses lead to late-phased changes in atmospheric CO 2 that provide positive feedback to the ice sheets and also project ‘late’ 41-K forcing across the tropics and the Southern Hemisphere. Responses in austral regions are also influenced by a fast response to summer insolation forcing at high southern latitudes. At the 22,000-year precession period, northern summer insolation again forces a lagged ice-sheet response, but with muted transfers to proximal regions and no subsequent effect on atmospheric CO 2. Most 22,000-year greenhouse-gas responses have the ‘early’ phase of July insolation. July forcing of monsoonal and boreal wetlands explains the early CH 4 response. The slightly later 22-K CO 2 response originates in the southern hemisphere. The early 22-K CH 4 and CO 2 responses add to insolation forcing of the ice sheets. The dominant 100,000-year response of ice sheets is not externally forced, nor does it result from internal resonance. Internal forcing appears to play at most a minor role. The origin of this signal lies mainly in internal feedbacks (CO 2 and ice albedo) that drive the gradual build-up of large ice sheets and then their rapid destruction. Ice melting during terminations is initiated by uniquely coincident forcing from insolation and greenhouse gases at the periods of tilt and precession.

Glaciers and Ice Sheets Mapping Orbiter concept

We describe a concept for a spaceborne radar system designed to measure the surface and basal topography of terrestrial ice sheets and to determine the physical properties of the glacier bed. Our primary objective is to develop this new technology for obtaining spaceborne estimates of the thickness of the polar ice sheets with an ultimate goal of providing essential information to modelers estimating the mass balance of the polar ice sheets and estimating the response of ice sheets to changing climate. Our new technology concept employs VHF and P‐band interferometric radars using a novel clutter rejection technique for measuring surface and bottom topographies of polar ice sheets from aircraft and spacecraft. Our approach will enable us to reduce signal contamination from surface clutter, measure the topography of the glacier bed at better than 1 km intervals with an accuracy of 20 m, and paint a picture of variations in bed characteristics. The technology will also have applications for planetary exploration including studies of the Martian ice caps and the icy moons of the outer solar system. Through the concept developed here we believe that we can image the base and map the three‐dimensional basal topography beneath an ice sheet at up to 5 km depth.

The Hinlopen Slide: A giant, submarine slope failure on the northern Svalbard margin, Arctic Ocean

New swath bathymetry data unveil a giant, submarine landslide located at the mouth of the Hinlopen transverse trough on the northern Svalbard margin, Arctic Ocean. Despite the relatively small drainage and slide scar areas, the Hinlopen Slide is exceptional in volume, headwall height and dimensions of the rafted blocks. From the c. 2200 km 2 large headwall area, approximately 1350 km 3 of Plio-Pleistocene sediments have been evacuated from the continental margin to the Nansen Basin. The escarpment heights are unprecedented, exceeding 1400 m, whereas the rafted blocks observed in the intermediate part of the slide area are up to 450 m high and more than 5 km wide. These characteristics make the Hinlopen Slide one of the largest landslides worldwide, and the first mapped mega-slide in the Arctic Ocean. Within the amphitheatre-shaped slide scar area, a composite set of escarpments and multiple, roughly planar slip surfaces occur, as well as detached sediment ridges adjacent to the outer escarpment, arcuate pressure ridges in debris lobes and isolated slump debris or blocks. From this complex slide scar geomorphology, we infer that the Hinlopen Slide was a translational, multi-phase slope failure that developed retrogressively. The slide has not been dated yet; however, the geophysical data suggest a relatively young age, though probably pre-Holocene, inferred from the pattern of sediment infill within the slide scar area derived from the Hinlopen cross-shelf trough. A submarine landslide with these dimensions could have created a devastating tsunami, although its potential depends on sea ice conditions as well as the time lags of the multi-phase, retrogressive movement. Climate changes and the response of ice sheets, ocean-current systems, sediment delivery and seismicity related to glacio-isostatic deformation appear as critical factors controlling stability on glaciated margins.

GRANTISM: An Excel TM model for Greenland and Antarctic ice-sheet response to climate changes

Over the last decades, the response of large ice sheets on Earth, such as the Greenland and Antarctic ice sheets, to changes in climate has been successfully simulated with large-scale numerical ice-sheet models. Since these models are highly sophisticated, they are only applicable on the scientific level as they demand a large amount of CPU time. Based on similar physics, a computationally fast flowline model of the Greenland and Antarctic ice sheet is presented here, primarily designed for educational purposes. Using an over-implicit numerical scheme, the model runs fast and behaves in a similar way to changes in background temperature forcing as major ice-sheet models do. A user-friendly interface and the implementation within a common spreadsheet program (Excel TM) make the model suitable for the classroom.

Sensitivity of Cenozoic Antarctic ice sheet variations to geothermal heat flux

The sensitivity of long-term Cenozoic variations of the East Antarctic ice sheet to geothermal heat flux is investigated, using a coupled climate–ice sheet model with various prescribed values and patterns of geothermal heat flux. The sudden growth of major ice across the Eocene–Oligocene boundary (∼34 Ma) is used as a test bed for this sensitivity. A suite of several million year-long simulations spanning the transition is performed, with various geothermal heat flux magnitudes and spatial distributions reflecting current uncertainty. The climate–ice sheet model simulates the Eocene–Oligocene transition realistically as a non-linear ice-sheet response to orbital perturbations and a long-term gradual decline of atmospheric CO 2. It is found that reasonable variations of geothermal heat flux have very little effect on overall ice volumes and extents, and on the timing of major ice transitions. However, they cause large changes in basal areas at the pressure melting point at a given time, which could strongly influence other aspects of Cenozoic Antarctic evolution such as basal hydrology, sediment deformation and discharge, subglacial lakes, and basal erosional forms.

Antarctic timing of surface water changes off Chile and Patagonian ice sheet response.

Marine sediments from the Chilean continental margin are used to infer millennial-scale changes in southeast Pacific surface ocean water properties and Patagonian ice sheet extent since the last glacial period. Our data show a clear "Antarctic" timing of sea surface temperature changes, which appear systematically linked to meridional displacements in sea ice, westerly winds, and the circumpolar current system. Proxy data for ice sheet changes show a similar pattern as oceanographic variations offshore, but reveal a variable glacier-response time of up to approximately 1000 years, which may explain some of the current discrepancies among terrestrial records in southern South America.

The role of greenhouse gases in orbital‐scale climatic changes

A decade ago, the Spectral Analysis, Mapping and Prediction Project (SPECMAP) analyzed the relationship between changes in Earth's orbit produced by variations in obliquity precession, and eccentricity and all available long‐term responses observed within the climate system [Imbrie et al., 1992, 1993]. In the intervening decade, improvements to the gas time scale for the Vostok ice core from Antarctica have provided age constraints on greenhouse gas changes that are comparable to those for climatic responses based on marine δ18O signals; these prompted a new investigation of the orbital‐scale climatic changes.Milutin Milankovitch proposed that variations in northern hemisphere summer radiation at the 22,000‐yr precession cycle and 41,000‐yr obliquity cycle drive northern ice sheets by summer ablation control of ice mass balance, and he inferred that ice sheet responses should lag 5000 years behind summer radiation forcing. In confirmation of Milankovitch's hypothesis, Hays et al. [1976] found that coherent changes in δ18O lag several thousand years behind the forcing at those two orbital cycles. Imbrie et al. [1984] determined that δ18O lags of 5000 years behind precession and 8000 years behind obliquity simultaneously satisfied a 17,000‐yr time constant of ice response (Figure 1a). These studies also detected a dominant δ18O response at 100,000 years that Milankovitch had not anticipated because direct solar forcing at that period is negligible. Because the 100,000‐yr δ18O signal is phased with orbital eccentricity, Hays et al. [1976] called it a “paced” response but left its origin unresolved.

A quasi-analytical ice-sheet model for climate studies

Abstract. A simple quasi-analytical model is developed to study the response of ice-sheets to climate change. The model is axisymmetrical and rests on a bed with a constant slope. The mechanics are highly parameterised. The climatic conditions are represented by the altitude of the runoff line. Above the runoff line the accumulation rate is constant (but may depend on, for instance, the ice-sheet size), below the runoff line the balance gradient is constant. The ice-sheet may extend into the sea and can respond to changes in sea level. At the grounding line the ice velocity is assumed to be proportional to the water depth. For this set-up an explicit expression for the total mass budget of the ice-sheet is derived. To illustrate the properties and possibilities of the model, equilibrium states are analysed and the response to periodic forcing is studied as well. The coupling of mass balance and surface elevation of the ice-sheet leads to nonlinear behaviour and branching of the equilibrium solutions. The qualitative behaviour of the system is that of the cusp catastrophe. Nonlinear effects are more pronounced when the slope of the bed is smaller. A case is discussed in which two ice-sheets are coupled by making the altitude of the runoff line dependent on the total area of the two ice-sheets. On two continents, having a slightly different glaciation threshold, periodic forcing of the altitude of the runoff line is imposed. It is shown that in such a situation variations on a long time scale (two to three times the period of the forcing) are introduced. Finally the model is forced by the GISP d18O record for the last 120 000 years. For an appropriate choice of parameters the model simulates well the waxing and waning of the Laurentide, Fennoscandian and Barentsz ice-sheets.

Climate friction and the Earth's obliquity

SUMMARY We have revisited the climate friction scenario during the Earth’s major glacial episodes of the last 800 Myr: the Late Pliocene-Pleistocene (∼0–3 Ma), the Permo-Carboniferous (∼260– 340 Ma) and the Neoproterozoic (∼750 ± 200 Ma). In response to periodic variations in the obliquity, the redistribution of ice/water mass and the isostatic adjustment to the surface loading affect the dynamical ellipticity of the Earth. Delayed responses in the mass redistribution may introduce a secular term in the obliquity evolution, a phenomenon called ‘climate friction’. We analyse the obliquity–oblateness feedback using non-linear response of ice sheets to insolation forcing and layered models with Maxwell viscoelastic rheology. Since the onset of the Northern Hemisphere glaciation (∼3 Ma), we predict an average drift of only ∼0.01 deg Myr −1 modulated by the main ∼1.2 Myr modulating obliquity period. This value is well reproduced when high-resolution oxygen-isotope records are used to constrain the ice load history. For earlier glaciations, we find that the climate friction effect is not proportional to the amplitude of the ice-age load, as it was previously assumed. A possible increase in the non-linear response of ice sheets to insolation forcing and latitudinal changes in this forcing may strongly limit the contribution of the obliquity variations to glacial variability, and thereby the climate friction amplitude. The low-latitude glaciations of the Sturtian glacial interval (ca 700–750 Ma) have probably no influence on the obliquity, while we predict a maximal possible absolute change of ∼2 ◦ for the Varanger interval (ca 570–620 Ma). We show that this mechanism cannot thus explain a substantial and rapid decrease in obliquity (of ∼30 ◦ ) as previously suggested by D.M. Williams et al. (1998) to support the high obliquity scenario of G.E. Williams (1993). Overall, we find that climate friction cannot have changed the Earth’s obliquity by more than 3–4 ◦ over the last 800 Myr.